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STARWATCH

Big and Little Dippers mark summer’s midpoint

At 3 p.m. Monday we’ll be exactly halfway through the summer. But summer certainly feels more than half over already.

It is not your imagination. We divide the seasons officially by the sun’s solstices and equinoxes, but this scheme offsets summer and winter pretty far from the yearly cycles of temperature, weather, and sunshine. The hottest days of the year come only a quarter of the way into our modern summer, and we get through the coldest days only a quarter of the way through winter.

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The offset of the seasons from daylight is even greater. The June 20 solstice, our official start of summer, marks the middle of the season of longest days. That’s why we still call it “Midsummer’s Night,” a holdover from the seasonal divisions used in Shakespeare’s time and before.

By that standard, the seasons changed at turning points that are now almost forgotten: the so-called “cross-quarter days.” These were marked (approx­imately) by ancient ­European festivals whose remnants we know as Halloween, Groundhog’s Day, May Day, and the virtually unknown ­Lammas, from the Anglo-Saxon for “loaf-mass” or “bread-feast,” the end of sunlight-summer and the start of harvest time: approximately now.

So, you may ask, what are the real seasons?

Polaris isn’t exactly dazzling, but you can always find it using the Big Dipper [now dipping in the northwest as shown here].

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Why, any you care to make up! Our four astronomical ones are as good as any. Many cultures divided the year into three: growing, harvest, and winter. Parts of the tropics recognize two: rainy and dry. When I lived in northern ­Vermont, people spoke of six: spring, summer, fall, shutdown, winter, and mud, of which winter was by far the longest.

The wheeling of the Dippers

The changing seasons are written smoothly in the starry night sky, with no obvious divisions at all. The night side of the world swings steadily around to face different parts of starry space throughout Earth’s annual circle around the Sun.

For instance, bright Vega now shines nearly straight overhead after nightfall. Look for it there.

Vega is one of the two brightest stars of summer. The other is Arcturus, now moving down the western sky. Altair is high in the southeast at dusk, and orange Antares glitters low in the south-southwest. That’s early August for you.

The earth’s axis of rotation points to one special place in the sky: the north celestial pole. This is straight overhead as seen from Earth’s North Pole, and as viewed from New England, it’s about halfway up the northern side of the sky. It’s closely marked by Polaris, the North Star. The entire celestial sphere seems to rotate around this point, as earth turns through the night and through the year.

Polaris isn’t exactly dazzling, but you can always find it using the Big Dipper (now dipping in the northwest as shown here). The front two stars of the Big Dipper’s bowl point nearly to it. Polaris is a little more than a Dipper-length away from them.

Polaris is also the handle-end of the dim Little Dipper. Most of the Little Dipper’s stars are too faint to see through the light-polluted skies inside Route 128. But at the Little ­Dipper’s other end is Kochab, Polaris’s equal, which at least shows how it’s supposed to be oriented.

People tend to assume there must be some natural law that puts a pole star at the celestial pole, but no. We’re just lucky. Hold a pencil at arm’s length, and its width shows how narrowly Polaris is separated from the exact point. Pure coincidence.

And it won’t last. Earth’s axis is slowly moving with respect to the fixed stars (though hardly at all with respect to earth’s surface), and because of this, during any given random millennium the celestial pole is usually bereft of a good north star. When Shakespeare had Julius Caesar declare, “I am constant as the northern star, of whose true-fixed and resting quality there is no fellow in the firmament,” the North Star was in fact circling around the pole point four times more widely than it does today.

In Caesar’s own time, Polaris was displaced from the pole four times farther still: by more than a fistwidth at arm’s length. It was not much of a pole star to the Romans at all, something Shakespeare either did not know or assumed his audience would not.

The north celestial pole will pass its closest to Polaris in March 2100, when it will be ­only a little closer than now. Then it will gradually move away. Earth won’t again have another pole star this bright ­until it’s Vega’s turn around 14,000 AD. And even then the lineup won’t be as good. When we look poleward now, maybe we should feel a bit special.

Whole-sky maps

Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/
gettingstarted.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (Skyand­Telescope.com). His Star Watch column appears the first Saturday of every month.

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